After delays put pay to its first attempt last week, Solar Impulse 2 is finally on its way to Abu Dhabi.

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Pilot Bertrand Piccard – who has now recovered from the illness which prevented the initial flight to take off on July 18 – left Cairo at 11:28pm UTC last night (1:28am CEST/7:28pm EDT on July 24). Si2 is expected to land in Abu Dhabi at around 11pm UTC tomorrow, Monday July 25.

The team said it had been difficult for the mission engineers to find a favourable window after they had to postpone the first attempt and Piccard now faces new flight conditions not seen before, including extreme heat, updrafts and downdrafts.

What it's really like to fly Solar Impulse 2: exclusive video reveals all about life on board

Over Saudi Arabia, the thermals will reach up to 21,000ft meaning the pilot will have to travel at high altitudes in an unpressurised and unheated cockpit for longer than usual.

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Elsewhere, the electronics on board don't have an active cooling system, and therefore they rely on the air to cool down.

The highest heat Si2 will encounter on this flight is 35 degrees celsius at the landing in Abu Dhabi. Due to the heat that comes from a Foehn wind originating in the Omani mountains, the team has to land at nighttime in order to handle the temperature on the ground and due to wind conditions.

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Solar Impulse

It's been almost 16 months since the solar-powered plane first left Abu Dhabi. Si2 had hoped to head back to where it all began on July 18, but both strong winds and Piccard's stomach upset pushed this back by a week.

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In a blog post announcing the delay last week, the team wrote: "We always put safety first and therefore have decided to postpone the flight and wait for [Piccard] to feel better in order to identify another weather window.

"The engineers at the Mission Control Centre in Monaco will continue to work hard to identify another window in the coming days."

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Relive the best photos from Solar Impulse 2's round-the-world trip

"We are moved by this flight," explained the project on a blog post. "A continuous criss-cross between thrill and sadness, we are finding it difficult to balance our emotions to prepare for our final landing with Si2 on this round-the-world tour."

In an exclusive video recently filmed for WIRED during the plane's Atlantic crossing, pilot Piccard revealed more about the team's excitement at the upcoming completion of the round-the-world trip. He gave a unique and personal insight into what it's really like to fly in the coffin-sized cockpit for days at a time.

The tiny cockpit – which measures just 40.9 square feet – has room for instruments, a small amount of food and a reclining chair. Pilots can have a daily food allowance of 2.4kg and 2.5 litres of water. The pilots are only able to sleep for 20 minutes at a time, and alarms make sure they don't doze for longer.

The plane's specifications

Solar Impulse 2 (Si2) can reach maximum speeds of up to 87mph, though its average speed is around 50mph. This may not sound fast but because it is powered by solar power rather than fuel, it can fly for days – or, theoretically, forever.

It left Abu Dhabi in March 2015 and is being flown on its 22,000-mile (35,000km) round-the-world journey by Swiss entrepreneur Borschberg and psychiatrist Piccard.

Solar Impulse

The journey has previously included a five-night and five-day journey from Japan to Hawaii, which at the time broke a record for the longest uninterrupted journey in aviation history.

Following its Atlantic crossing, it flew through Europe and on to the Middle East.

The plane weighs little more than a car, but has the wingspan of a Boeing 747. The aircraft is powered thanks to 17,000 solar cells, and it runs on battery power during the night.